Expert: Arshad Khan - 3/22/2010

Question
I am an Assistant RE on one construction site in Malaysia. Recently the RC driven pile refuse to set at our expected depth. After two extra joints of 6m lengths we were convinced that the pile has given way and failed to set and I have decided to abandon that pile.

I therefore instructed the piling contractor to pile an additional pile about 250mm away from the abandoned pile to act as a replacement.  I also have to redesign the pile cap and possibly the stump as well.  It is a 3pile cap. I have found that the centroid of the pile reaction has moved to about 150mm away from the original centroid.  Upon calculating the reaction of new pile configuration the original piles have to carry more of the loads from the stump which is slightly more than the designed pile load capacity of 300Kn per pile. I attempted to increase the size of the stump width in my hope to move the centre of gravity of the stump load nearer to the centroid of the pile reaction which would spread the stump load almost equally amongst the piles which could reduce the loading in two of the piles which were more than 300Kn to below acceptable level.

The stump is 1500mm height and the new width of the stump is 650mmX350mm while the previous stum is 300mmX300mm.

My question, is the stump capable to spread the eccentric load at the top of the stump to the cente of the stump crossection at the bottom of the stump i.e at top of pile cap? If it ca capable then my design is OK.

Answer
Hi Ahmad,

If the column vertical reaction remains on the position of the original 300x300 stump, then increasing the stump width will not significantly alter the position of the resultant vertical force on to the pile cap, I am afraid!

You have 2 choices- either accept that the piles will be slightly over-loaded (i.e. factor of safety will be slightly reduced form 2.5 or 3), or else you will need to install additionl pile(s) to compensate.